In the transport refrigeration industry, an internal combustion engine, such as a diesel engine, commonly drives a refrigerant compressor at a selected one of two operating speeds, such as 1400 RPM and 2200 RPM, respectively called "low" and "high" speed. The speed selection is determined automatically by a temperature control algorithm. Thus, the percentage of operating time at each engine speed depends upon a plurality of factors, such as the characteristics of a load in a cargo space being conditioned by the transport refrigeration unit, the set point temperature of the conditioned space, the ambient temperature, the opening of an access door to the conditioned space during cargo transfers, and the like. Simply changing the engine oil based upon a fixed number of operating hours of the engine would result in a waste of oil when the oil change interval is too short, and it would reduce the useful operating life of the engine when the oil change interval is too long.
Thus, it would be desirable, and it is an object of the invention, to provide a new and improved method of determining oil change intervals, based upon the actual operating conditions of the engine during each measured oil change interval. It would also be desirable, and is another object of the invention to provide a visible indication or alarm, when the method determines that an oil change should be made.